Gallium nitride (GaN) and related III-V nitride alloys are wide bandgap semiconductor materials that have applications in opto-electronics (e.g., in fabrication of blue and UV light emitting diodes and laser diodes) and in high-frequency, high-temperature and high-power electronics. In such high-performance devices, high quality epitaxial films must be grown on the substrate.
Gallium nitride-based electronic devices are typically grown on foreign (heteroepitaxial) substrates such as sapphire and silicon carbide. Due to the resultant mismatch of lattice constants and thermal expansion differences between the gallium nitride device layers and the foreign substrate, a high density of defects typically is produced in the gallium nitride device layers, which in turn adversely affects device performance.
Growth of gallium nitride device layers is typically performed by metal-organic chemical vapor deposition (MOCVD) or metal-organic vapor phase epitaxy (MOVPE), with a buffer layer first being grown on the foreign substrate, followed by growth of a few microns thickness of gallium nitride and associated device layers. To reduce crystal defects in the gallium nitride layer, techniques such as epitaxially laterally overgrown (ELOG) growth have been employed on sapphire or silicon carbide.
In view of the morphological and structural deficiencies associated with use of heteroepitaxial substrates, native gallium nitride substrates would be ideal for many gallium-nitride based microelectronic devices. Gallium nitride substrates can be prepared by various methods.
Porowski et al. U.S. Pat. No. 5,637,531 describes growth of bulk gallium nitride from metallic gallium at high nitrogen pressure, but the disclosed method has achieved maximum crystal size of only about 10 mm platelets (S. Porowski and I. Grzegory, J. Cryst. Growth, vol. 178, 174 (1997), M. Bockowski, J. Cryst. Growth, vol. 246, 194 (2002)). Gallium nitride crystalline platelets are c-plane structures and have polar surfaces, with one face of the platelet terminated with gallium and the other face terminated with nitrogen. Each of the respective surfaces has distinct properties, and most gallium nitride-based devices are as a matter of preference grown on the gallium-terminated surface, i.e., the (0001) surface. Although the size of the crystal platelets is small, homoepitaxial growth has been carried out on samples of such platelets. For example, MOVPE homoepitaxy has been carried out on gallium nitride crystalline platelets with lateral dimensions of less than 5 mm (F. A. Ponce, D. P. Bour, W. Götz and P. J. Wright, Appl. Phys. Lett., 68(1), 57 (1996)). High electron mobility transistor (HEMT) structures based on AlGaN/GaN heterostructures have been grown on 8×8 mm2 gallium nitride samples by molecular beam epitaxy (E. Frayssington et al., Appl. Phys. Lett. 77, 2551 (2000)). InGaN/GaN multiple quantum well (MQW) structures and double heterostructure LEDs have been grown on approximately 6×8 mm2 gallium nitride samples by MOVPE (M. Kamp et al., MRS Internet J. Nitride Semicond. Res. 4S1, G.10.2 (1999)). MOVPE homoepitaxial growth on nitrogen-terminated gallium nitride (0001) crystal platelets and on surfaces slightly tilted away from the (0001) plane has been reported (A. R. A. Zauner et al., J. Crystal Growth, 210, 435 (2000)).
Since the manufacture of opto-electronic and electronic devices requires large area substrates, various devices have been grown on large-area gallium nitride substrates produced by other techniques. In one such technique, gallium nitride-based laser diodes have been fabricated (S. Nakamura, et al., Jpn. J. Appl. Phys. 37, L309 (1998)) by a complicated growth sequence. First, a 2 micron thick MOVPE gallium nitride layer was grown on a sapphire substrate, followed by deposition of a 2 micron thick silicon dioxide layer patterned into stripes. A 20 micron thick gallium nitride layer then was grown by MOVPE using ELOG technique to cover the silicon dioxide pattern and achieve a flat gallium nitride surface. This was followed by hydride vapor phase epitaxy (HVPE) to form a gallium nitride layer of about 100 microns thickness. Next, the sapphire substrate was removed by polishing to obtain a gallium nitride article of about 80 microns thickness. Finally, an InGaN MQW LD structure was grown by MOVPE.
Ogawa et al. U.S. Pat. No. 6,455,877 discloses growth of light emitting devices on gallium nitride substrate formed by HYPE deposition of gallium nitride on ELOG gallium nitride formed by MOVPE on sapphire, wherein the sapphire was polished away after formation of sufficient gallium nitride thickness. Ogawa et al. describes a preferred substrate orientation of 0.10 to 0.25 degree tilt away from the c-plane of the gallium nitride material. In subsequent U.S. Published Patent Application No. 2001/0030329, Ueta, et al. state a preference for a substrate orientation of 0.05-2 degrees tilted away from the c-plane of the gallium nitride material. In these various device structures, the device layer was grown by MOVPE directly on the as-grown HYPE gallium nitride surface.
U.S. Patent Publication No. 20050104162 discloses a GaN substrate including a GaN (0001) surface off-cut from the (0001) plane predominantly towards a direction selected from the group consisting of <10 10> and <11 20> directions and methods for making the same. GaN substrates with surfaces intentionally tilted away from the lattice c-plane (0001), i.e., wafers with vicinal c-plane surface or with off-cut surfaces, exhibit surface lattice step structures. Furthermore, the intentionally tilted surfaces of GaN substrates allow homoepitaxial growth to be carried out which produces high quality smooth homoepitaxial films of GaN.
As known to those familiar with photonic devices such as LEDs and lasers, the frequency of electromagnetic radiation (i.e., the photons) that can be produced by a given semiconductor material is related to the material' s bandgap. Smaller bandgaps produce lower energy, longer wavelength photons, while wider bandgap materials produce higher energy, shorter wavelength photons. For example, one semiconductor commonly used for lasers is aluminum indium gallium phosphide (AlInGaP). Because of this material's bandgap (actually a range of bandgaps depending upon the mole or atomic fraction of each element present), the light that AlInGaP can produce may be limited to the red portion of the visible spectrum, i.e., about 600 to 700 nanometers (nm). In order to produce photons that have wavelengths in the blue or ultraviolet portions of the spectrum, semiconductor materials having relatively large bandgaps may be used. Group III-nitride materials such as gallium nitride (GaN), the ternary alloys indium gallium nitride (InGaN), aluminum gallium nitride (AlGaN) and aluminum indium nitride (AlInN) as well as the quaternary alloy aluminum gallium indium nitride (AlInGaN) are attractive candidate materials for blue and UV lasers because of their relatively high bandgap (3.36 eV at room temperature for GaN). Accordingly, Group III-nitride based laser diodes have been demonstrated that emit light in the 370-420 nm range. Published U.S. Application Nos. 20040152224, 20040147094, 20040147054, and 20040149997 describe various methods and structures for gallium-nitride based laser devices.
The contents of all of the foregoing patents, patent applications and published patent applications are incorporated entirely herein by reference as if fully set forth herein.